The microelectronics industry strives toward fabricating high density circuitry by decreasing the minimum feature size of the components on the chip. This requires high-resolution lithography, the principal technique used in patterning microelectronics circuitry. Over approximately the last 20 years, the industry has migrated to shorter wavelength photolithography as the primary means of scaling the resolution to sustain the progressive demand for smaller features. The wavelength of photolithography has migrated from mid-ultraviolet (MUV) wavelengths (350450 nm) to deep-UV (DUV) radiation (190–300 nm) and vacuum UV (VUV, 125–160 nm). Likewise the photosensitive materials used in photolithography, photoresists, have evolved. MUV lithography employed diazonaphthoquinone (DNQ) and novolak-based resists. These materials offered high performance but were not extendible to DUV and VUV wavelengths due to their opacity at these shorter wavelengths. In addition, these resists were not of sufficient sensitivity to afford high throughput manufacturing.
In response to the need for new, lower opacity, higher sensitivity materials for DUV imaging, Ito et al. disclosed (U.S. Pat. No. 4,491,628) the development of chemically amplified resists (CARs) based on the photochemically-generated-acid (from a photosensitive acid generator) catalyzed deprotection of acid-labile polymer. That is, for positive tone CARs, labile moieties of the polymer are cleaved by acid catalyzed thermolysis reaction that renders the deprotected form of the polymer soluble in a subsequently applied developer, such as aqueous base. Thus, an image of the projected patternwise radiation is formed in the resist film after development, which can then serve as an etch-resistant mask for subsequent pattern transfer steps. The resolution obtained is dependent on quality of aerial image and ability of resist to maintain that image.
CARs have been developed for 248, 193, and 157 nm lithography. The theoretical dimensional limit of equal-sized half-pitch features is one quarter of the wavelength, Î>>(k1=0.25) when NA=1, as the dose applied to the resist is equal to the square of the intensity, and thus the resolution cannot be modulated by any more than Î>>/4, or a pitch of Î>>/2. The resolution attainable with each advancing generation of materials has been extended toward these limits through the use of low k1 techniques and high numerical aperture tools. For the latest VUV wavelength being developed for manufacturing, 157 nm, with a very high but potentially manufacturable NA of 0.95, Î>>/4 equals ˜40 nm. To obtain images below this feature size, either an extension of NA to >1, such as is afforded with immersion lithography or with a non-diffraction limited, non-optical lithography system, such the so-called next generation lithography (NGL) options. The most promising of these NGLs are extreme ultraviolet (EUV, sometimes referred to as soft x-ray) or electron beam lithography (EBL).
One barrier to imaging in the sub-50 nm half-pitch regime is a phenomenon known as image blur diminishes the integrity of the pattern (Hinsberg et al., Proc. SPIE, 2000, 3999, 148 and Houle et al., J. Vac. Sci. Technol B, 2000, 18, 1874). Image blur can be defined as the deviation of the developable image from that of projected aerial image which is transferred into the film as the concentration of photochemically generated acid. While accelerating the rate of the deprotection reaction, the application of thermal energy diminishes the fidelity of the aerial image of acid formed during the patternwise exposure. Image blur can be divided into two contributing factors: gradient-driven acid diffusion and reaction propagation. Both factors contribute to blur, but to different degrees and with different temperature dependence.
The first factor contributing to image blur is often referred to as acid diffusion and can be described by Fickian diffusion models for solids (Hinsberg, 2000). Choice of the photoacid being generated from photoacid generator (PAG) and the mobility in the chosen polymer matrix dictate this factor. The mobility in the polymer matrix is dependent on the comprising chemical functionality of the polymer, the free volume of the matrix, the glass transition temperature (Tg) of the polymer and the temperature and time of baking steps encountered during the resist processing.
A second contributing factor to image blur is sometimes described as reaction propagation (Hinsberg, 2000; Houle, 2000) and is best described by Arhenius behavior. Activation energy (enthalpy), volatility of products (entropy), and the availability and concentration of deprotection-reaction-dependent co-reactants such as moisture dictate the degree to which the reaction propagates away from the original acid profile.
In order to achieve high resolution, high sensitivity and high degree of process latitude, both image blur factors must be eliminated or minimized. Both of these contributing factors can be tempered by the addition of acid-quenchers, or bases, which have been shown to reduce image blur. Additionally it has been recognized that image blur is temperature dependent. Breyta et al. disclose that appropriate baking conditions can optimize the resolution attainable with CARs in U.S. Pat. No. 6,227,546. However, since the extent of thermally induced image blur has been estimated to be on the order of 10–50 nm with conventional resist processing schemes by various researchers (Hinsberg 2000; Houle 2000; Krasnaperova et al., J. Vac. Sci. Technol. B, 1994, 12, 3900; Lin et al, Science 2002, 297, 372), processing that reduces this phenomenon further are desirable and necessary to reach the sub-50 nm half-pitch regime of imaging.
Thus, remains a need for reliable methods of performing sub-50 nm half-pitch imaging.